Cavitation damage repair on pumps and turbines

Cavitation damage is repaired in three stages: thoroughly clean and blast the pitted areas, rebuild the lost material with a filled composite, then protect the rebuilt surface with an elastomer or a composite formulated to resist cavitation. The repair only holds if the hydraulic cause is corrected at the same time: otherwise cavitation returns at the same spot.

Cavitation is one of the fastest forms of damage on a pump or turbine. It leaves a rough, sponge-like surface that costs efficiency, unbalances the impeller and eventually holes the wall. This guide is for maintenance managers at plants, pump stations and generating stations who want to repair properly and avoid repeat damage. It complements our broader procedure for centrifugal pump epoxy repair.

How does cavitation destroy metal?

Cavitation destroys metal through collapsing vapour bubbles. Where local liquid pressure drops below its vapour pressure, bubbles form; when they reach a zone of higher pressure, they collapse violently. Each collapse near a wall produces a micro-jet and a shock wave that tear tiny particles from the surface.

Repeated at very high frequency, this hammering creates the deep pits and sponge-like look described in our cavitation damage glossary entry. Corrosion often speeds the process: each collapse strips the protective oxide layer and exposes fresh metal. Cavitation differs from particle erosion, which leaves grooves aligned with the flow, as our article on erosion-corrosion explains.

The affected zones are typical:

• Centrifugal pumps. Impeller eye, suction side of the vanes, cutwater, wear rings.
• Hydraulic turbines. Runner blades, especially toward the outlet, discharge ring, draft tube inlet.
• Valves. Seat and plug downstream of a heavy throttle.

Which hydraulic causes should be corrected first?

The hydraulic cause comes first, because cavitation is a symptom. The most frequent causes are too little margin between net positive suction head available (NPSHa) and the head required by the pump, operation far from the best efficiency point, and degraded suction conditions.

• NPSH margin. Suction level too low, clogged strainer, long or undersized suction line, partly closed suction valve, liquid hotter than expected. ANSI/HI 9.6.1 covers the NPSH margins to aim for.
• Operating point. A pump run at low flow develops recirculation at the impeller eye, which cavitates even when NPSH seems adequate. At high flow, required NPSH rises.
• Suction geometry. Elbow too close to the flange, vortexing in the sump, air ingress.
• Turbines. Operation outside the intended range, low tailwater, blade profile altered by successive repairs.

A survey of pressures, flow, level and temperature, compared with the pump curve, guides the diagnosis. Our maintenance and rehabilitation service coordinates this survey with the repair.

Which materials rebuild a cavitated surface?

A cavitated surface is rebuilt with two complementary families: a metal-filled or ceramic-filled epoxy composite to restore volume and geometry, and a more resilient protective layer, an elastomer or a composite formulated for cavitation, that absorbs the collapses. The table summarizes their roles.

Choosing the filler composite is explained in our guide to metal repair epoxy putty selection. The relative cavitation resistance of materials is assessed in the lab, for example to ASTM G32 (vibratory apparatus) or ASTM G134 (cavitating jet); rely on the manufacturer's published results for these tests, not on sales claims.

How should the surface be prepared before rebuilding?

The surface is prepared by high-grade abrasive blasting, down to the bottom of the pits, on a part that is clean, dry and free of salts. This is the hardest step, because cavitation leaves a surface riddled with cavities that hold oil, pumped liquid and salts.

1. Decontamination. Washing and degreasing, then heating or drying to draw out contaminants absorbed in the porous metal. Repeat until the surface stops sweating.
2. Salt check. On a part that pumped seawater, leachate or a chemical, soluble salts are measured to ISO 8502-6 and ISO 8502-9, then washed again if needed.
3. Blasting. Abrasive blasting to the specified degree, often near-white SSPC-SP 10 or white metal SSPC-SP 5, with an angular profile. Our abrasive blasting service works in the shop or on site.
4. Prompt application. The composite goes on the blasted surface before any recontamination or flash rust, above the dew point with the required margin.

Metal that is too thin or holed is reinforced first, for example with a bonded plate or a mesh embedded in the composite, before rebuilding.

How does the rebuild proceed?

The rebuild is done in layers: filling the pits with the rebuilding composite, shaping to the original profile, then applying the protective layer within the specified recoat window. The hydraulic profile matters as much as thickness: a poorly reshaped blade creates new low-pressure zones.

• Filling. The composite is pressed into each pit to drive out air, then built slightly above the final profile.
• Reshaping. Templates, dimensions taken before repair or a sound neighbouring part serve as reference. Blade leading and trailing edges are reworked with care.
• Protective layer. The elastomer or cavitation-resistant composite is applied at controlled thickness over exposed zones, overlapping onto sound metal.
• Curing. Full cure per the data sheet is reached before rewatering, with heating if needed.
• Balancing. A repaired impeller is rebalanced before reassembly.

The repair is documented: before and after photos, dimensions, products, batches, ambient conditions and thicknesses. This record allows comparison at the next inspection.

What are the limits of composite repair?

Composite repair has clear limits: it does not fix the hydraulic cause, it may not tolerate a high service temperature depending on the product, and it does not replace a structural rebuild when the impeller or blade has lost its mechanical strength. Very severe cavitation can also wear an elastomer faster than expected.

Cases where something else should be considered:

• Wall holed over a large area or cracked. A new part or a weld repair is safer.
• Hot or aggressive liquid. Chemical and thermal compatibility of the composite is checked on the data sheet before choosing.
• Rapid recurrence despite a careful repair. Almost always, the hydraulic cause was not resolved.

Our article on whether to repair or replace industrial equipment helps decide when the part is borderline. For a slurry pump, where abrasion adds to cavitation, see also slurry pump wear coatings.

How should the repair be planned during a shutdown?

The repair is planned from a prior inspection: photos, pit depth, affected area and condition of neighbouring parts. These data let you plan products, the drying method and cure time, which often set the length of the outage.

A removable pump impeller is easier to repair in the shop, where temperature and humidity are controlled. A turbine is repaired in place, with access, ventilation and heating organized in the pit or runner chamber, as for other hydroelectric coatings work. In every case, the repair fits within a protective coatings system consistent with the rest of the asset.

Frequently asked questions

Can a cavitated impeller be repaired instead of replaced?

Yes, as long as the impeller keeps its mechanical strength and the pits do not extensively pierce the wall. The composite restores volume and the elastomer protects the exposed zone.

Is epoxy alone enough against cavitation?

Rarely. A rigid epoxy at the surface cracks under repeated collapses. It is used to rebuild, then protected with a more resilient layer.

How do you tell whether cavitation comes from NPSH or recirculation?

Location helps: on the suction side of the vanes at the eye, it is often NPSH margin; on the pressure side or mid-vane at low flow, it is often recirculation. A survey of the operating point confirms it.

Can a cavitation repair be inspected?

Yes. Preparation, layer thickness and cure are checked, then the zone is inspected after a period of service to confirm the cause was resolved.

Our crews and qualified subcontractors repair pumps and turbines in the shop or on site, working from LaSalle, including in Mauricie and Centre-du-Québec and Saguenay-Lac-Saint-Jean. Send photos of the pitted zones, the equipment model, the pumped liquid and the operating point through the quote request: pricing on consultation, reply within 24 business hours.